Prof. Patrick Irwin

Abstract:

The Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft has been orbiting Saturn for 2-1/2 years. CIRS is a Fourier transform spectrometer that measures atmospheric thermal structure and dynamics, and atmospheric composition, of Saturn and Titan. CIRS also maps the temperatures and dynamical processes of the rings and icy moons. © 2007 Optical Society of America.

Abstract:

Prior to Uranus' northern spring equinox in 2007 December, near-infrared spectra and images of Uranus were obtained in 2006 August and September using the UIST instrument on the United Kingdom Infrared Telescope (UKIRT). Latitudinally resolved spectra were recorded between 1 and 2.5 μm at a resolving power varying between 550 and 2000 with the instrument in long slit mode and with the slit aligned with the planet's central meridian to determine the north-south variations of Uranus' cloud structure. Our observations appear to be the first latitudinally resolved complete (1-2.5 μm) near-IR spectra of Uranus ever recorded, and we present initial determinations of the latitudinal variation of Uranus' vertical cloud structure from these data. We find two main cloud decks of similar density, one based near the 2 bar level and one based in the 8-10 bar region. The upper cloud is found to extend from 50° south to 45° north, increasing in thickness and altitude toward the south, especially in the south circumpolar collar at 45° south, but clearing toward the poles. However, we find that the deeper cloud layer is thickest at the equator and thins symmetrically toward both poles. We also report the firstever observation that the bright south circumpolar collar is, at some wavelengths, actually darker than other latitudes and provide an explanation of this phenomenon in terms of the latitudinal variations in cloud structure. © 2007. The American Astronomical Society.

Abstract:

Thin-film transmission spectra of ammonium hydrosulfide (NH4SH) ice and ammonia (NH3) ice between 1300 and 12,000 cm-1 were used to determine the ice's optical constants. The films were grown on a sapphire substrate, and a Fourier-transform spectrometer and a grating spectrometer were used together to record the spectra. Lambert's law was used to directly determine the imaginary component of the complex refractive indices; from this, the real component was derived using the Kramers-Kronig algorithm. It is shown that, contrary to what is expected, the optical constants determined for NH3 ice at 80 K are in good agreement with those in the cubic phase, rather than the metastable one. The phase of the NH4SH ice was observed to change from amorphous to polycrystalline as the film was annealed to 160 K. © 2006 Optical Society of America.